In X-ray imaging, for example, in computer tomography, angiography or radiography, integrating indirectly-converting X-ray detectors or counting directly-converting X-ray detectors can be used.
The X-rays or the photons can be converted into light in indirectly-converting X-ray detectors using a suitable converter material, and into electric pulses, using photodiodes. Scintillators, for example, GOS (Gd2O2S), CsJ, YGO or LuTAG, are often used as a converter material. Scintillators are used in particular in medical X-ray imaging in the energy range up to 1 MeV. It is normal practice to use “indirectly-converting X-ray detectors”, in which the conversion of the X-rays or gamma rays into electric signal values takes places in two steps. In a first step, the X-rays or gamma quants are absorbed in a scintillator element and converted into optically visible light, an effect known as luminescence. The light excited by luminescence is subsequently converted in a second step into an electric signal or a signal value by a first scintillator element that is optically coupled to a first photodiode. The signal value can be read out in digitalized form via an evaluation and readout electronics unit and subsequently forwarded to a system calculating unit.
The X-rays or photons can be converted into electric signals or pulses in directly-converting X-ray detectors using a suitable converter material. As converter materials, it is possible to use, for example, CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs or others. The signals are evaluated by an evaluation unit, for example, an integrated circuit (Application Specific Integrated Circuit, ASIC). In counting X-ray detectors, incident X-rays are measured by counting the electric pulses that are triggered by the absorption of X-ray photons in the converter material. The height of the electric pulse is generally in proportion with the energy of the absorbed X-ray photon. As a result thereof, spectral information can be extracted by comparing the height of the electric pulse with a threshold value. The electric pulse can be converted into a signal value. Signal values can be read out and forwarded to the system computation unit.
An X-ray detector, in particular for use in a computer tomography unit, can consist of very many, for example, 1000 to 100,000, individual measurement channels, which generally operate simultaneously. Each measurement channel can comprise a collimation unit, a scintillator unit, a photodiode, a current/voltage converter, an analog-to-digital converter, and a computation unit with a plurality of steps. Typically, the collimation unit, the scintillator unit, the photodiode, the current/voltage converter, and the analog-to-digital converter are located in the detector system, that is, in the X-ray detector, in a detector group or in a detector apparatus. The computation unit is located outside the detector system, the signal values being forwarded by way of serial transmission, for example, in the Mbit/s or Gbit/s range, for example, to the system computation unit of the computer tomography unit.
The image quality or image impression can be improved by having a series of physical parameters, for example, the temperature coefficient or intensity linearity, operating homogeneously across the spatial extent or the course over time.